There is widely used an image forming apparatus, which is configured to apply a charging voltage, which is obtained by superimposing an AC voltage on a DC voltage, to a charging member (for example, charging roller), which is brought into contact with or near a circumferential surface of a rotating image bearing member, to thereby electrically charge the image bearing member. The AC voltage of the charging voltage has a peak-to-peak voltage that is equal to or more than twice an electric discharge start voltage between the image bearing member and the charging member, and electrically charges the circumferential surface of the image bearing member to a potential of the DC voltage of the charging voltage accompanying electric discharge between the image bearing member and the charging member.
When the AC voltage of the charging voltage is too high, overdischarge occurs, with the result that the surface of the image bearing member becomes rough, or a circumferential surface of the charging member is soiled. On the other hand, when the AC voltage of the charging voltage is too low, underdischarge occurs to impair uniformity of a charged state of the surface of the image bearing member, with the result that uneven density and a noise pattern are disadvantageously generated in an output image. Therefore, a setting mode for the AC voltage is executed before starting image formation or at intervals in the image formation to appropriately set the AC voltage of the charging voltage (Patent Literature 1).
In the setting mode described in Patent Literature 1, each of the AC voltage having the peak-to-peak voltage that is equal to or more than twice the electric discharge start voltage between the image bearing member and the charging member, and an AC voltage having a peak-to-peak voltage that is less than twice the electric discharge start voltage is applied to the charging member in a plurality of steps. Then, an AC current flowing through the charging member is measured in a state in which each AC voltage is applied, and a peak-to-peak voltage of an AC voltage of a charging voltage to be used during the image formation is set based on a measurement result of the AC current.